Bind your OPTS account to Social Network Service (SNS)
As of the app update on 23 June 2016, you can now bind your OPTS account to your social media account. Currently, the only option given is Facebook.
Update 27/10/2016: the option for Twitter is added.
Binding to your Facebook/Twitter
To do so, follow the steps here, but choose Facebook or Twitter. You can also un-bind your account later by going through the same steps, and bind it with a different FB/Twitter account.
Moving to another device
After binding, you can use the same steps to log-in to the game with your social media account if you are changing to a different device. (Note that your rainbow coins will still be reset to zero if it’s a move between iOS and Android, either direction.) Your OPTS account will be removed from the old device, so you can still only play on one device at one time. The good news is that you will not have to worry about 7-day expiry of one-time PIN and the maximum-twice-in-30-day transfer rule (note that you can still choose the one-time PIN as a method to transfer your account if you prefer not to use FB/Twitter).
To ‘log-in’, click on the button at the top left of the title screen and choose the top button of the right column, choose Yes, then you will see Facebook, Twitter, and PIN options.
Then choose Facebook or Twitter and key in your FB/Twitter details. Do not use Private Browsing.